These cooperative systems comprise a so-called secondary surveillance radar and airborne transponders on board cooperative carrier vehicles, such as aircraft. The secondary radar cooperates according to a determined protocol with the transponders (also called responders). The secondary radar comprises an interrogator which emits interrogation pulses modulated in amplitude and in phase at the frequency of 1030 MHz to establish a communication with the transponders present in the emission lobe of its antenna. These transponders respond via trains of pulses modulated in amplitude at the frequency of 1090 MHz. These pulses are received and processed by a receiver of the secondary radar.
The secondary radars used in this application are called surveillance radars (known as “Secondary Surveillance Radar” or SSR). The International Civil Aviation Convention, also called the ICAO standard (International Civil Aviation Organization) defines a communication protocol for secondary radars in its annex 10 (Aeronautical Telecommunications), volume IV (Surveillance radar and anti-collision systems). The ICAO standard defines several interrogation modes, such as modes A, C and S. Mode S is distinguished from modes A and C in that it allows selective interrogation of aircraft by the use of an identification number specific to each aircraft.
A mode S response is composed of a preamble formed of a series of four pulses whose positions are invariable, followed by a data block composed of 56 or 112 binary values or bits, the binary values being coded over successive time intervals of 1 μs, by pulse position modulation. The last 24 bits of the data block form a cyclic redundancy code (CRC) and the other bits form a message. The CRC allows error-free decoding of the message when it has experienced a deterioration over a zone of less than 24 μs.
It turns out that the deterioration of the message often exceeds 24 μs in particular in cases of multipaths (where it can impinge on the entire message) and in cases of nesting of a response in mode S with several responses in conventional mode A or C.
Multipath phenomena occur frequently in the terminal zone when the aircraft is near the ground and near the interrogator of the secondary radar. They are due to reflections of the response of the transponder off buildings and off the ground which arrive shifted over time with respect to the direct response and perturb the latter.
French patent application No. 89 14416 “device for detecting signals of responders interrogated by a secondary radar in the presence of multipath phenomenon” (FR 2.654.217) filed on Nov. 3, 1989 describes a method for determining the average power of a mode S response by analyzing the histogram of the values of the possible pulses for the duration of a mode S response. This method is implemented by a device for detecting data pulses allowing the decoding of mode S responses in the presence of multipath phenomenon.
More precisely, the position of the preamble pulses is used to generate timing signals. The timing signals make it possible to gather three samples of the reception signal per interval of 0.5 μs, that is to say per potential position of a pulse of the data block. A first sample is taken in the middle of the interval, and the other two flank the first. The values of the first samples are plotted in a histogram, with a weighting calculated on the basis of the other two samples. A reference value of the mode S response is then determined on the basis of the histogram, the reference value reflecting the amplitude level of the data pulses of the mode S response. A data pulse is detected when the value of the sample in the middle of an interval of 0.5 μs is close to the reference value.
However, if the mode S response is nested with another mode S response, a majority of the samples is impaired. This affects in particular the samples taken in the middle of the intervals of 0.5 μs, whose values are used not only to detect individual data pulses, but also to determine the reference value. Consequently, this method is unsuitable in the event of nesting of two mode S responses, and more generally in the event of pollution by a multipath or by a nesting with a signal whose power is of the same order of magnitude or greater than the power of the response under analysis.